[mylesdw]

The Le Mans style flip top filler cap screws onto a cast ally flange with a 3.5" bsp thread. This cast flange in turn bolts with a circle of six machine screws to another flange in mild steel to which the flexible coupling from the tank filler attaches.

BA is short for British Association Standard Thread and was the recommended thread for fasteners less than 1/4" before metric came along. 2BA has a major diameter 4.7mm and approx 31.4 tpi. The thread angle is 47.5. The steel flange has captive nuts that I don't really want to change.